Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long life span and low self-discharge are commercially available and widely used.
The lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like as the anode active material have been considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle life, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life.
In recent years, methods to use lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a NASICON structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known NASICON structure compound, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely known olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high output voltage of 3.5 V and a high theoretical capacity of 170 mAh/g, as compared to lithium (Li), and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe as an ingredient, thus being highly applicable as the cathode active material for lithium secondary batteries. However, such an olivine-type LiFePO4 has an operational efficiency of about 100%, thus making it difficult to control with the operational efficiency of an anode.
In this regard, by imparting equivalent operational efficiency to a cathode and an anode in batteries, inefficient waste of the electrodes can be minimized. For example, in the case where an anode having efficiency of about 100% is used for a battery, the battery can exert 100% efficiency, while when a cathode having 100% efficiency and an anode having 90% efficiency are used for a battery, the battery can exert only 90% efficiency. As a result, 10% of the efficiency of the cathode is disadvantageously wasted.
For example, in the case of generally-used carbon-based anode active materials, about 10-20% irreversible capacity are generated upon initial charge/discharge including the first charge and its reversible capacity is only about 80 to 90%. Accordingly, when a material having an efficiency of 100% is used as a cathode active material, the electrode material is disadvantageously wasted in direct proportion to the irreversible capacity of about 10 to 20%. In addition, when an anode active material having relatively low efficiency is used, an amount of anode active material used should be increased, depending on a higher efficiency of a cathode, which disadvantageously entails an increase in manufacturing costs.
On the other hand, in order to impart 100% efficiency to a battery using a cathode having 100% efficiency, an anode having about 100% efficiency should be used. In this case, the selection range of an anode active material is disadvantageously narrowed.
However, to date, there is no technology suggesting a method for controlling efficiency of LiFePO4 as a cathode active material.
In addition, there is an increasing need for a breakthrough that can considerably improve electrical conductivity of LiFePO4 and solve Li+ diffusion problems thereof via improvement in initial IR drop and Li+ diffusion properties.